|Section 4 – Dosimetric quantities
(i) Title and first paragraph - “dosimetric quantities” is not the best title or correct term to use and could be confusing. “Dosimetric quantities” is a term specifically applied by ICRU to the quantities kerma, exposure, cema, energy deposit, energy imparted, lineal energy, specific energy and absorbed dose. The term does not apply to dose equivalent quantities and protection quantities.
(ii) Effective dose: the details of the definitions of dosimetric, dose equivalent, operational and protection quantities have caused problems for over 20 years. Also it is over 15 years since the first discussions on the definition of effective dose. After exposure to many different arguments for and against its use the following reasons are given as to why the replacement of effective dose equivalent by effective dose was not constructive.
(a) We base our assessment of the relative effects of different radiation fields/particle types and energies, on the different microscopic patterns of energy deposition as this translates into different lesions and chemical species produced. This might be done with full computer simulation, or with some simplified track structure approach, but in most radiobiology this is still done in terms of LET. (This latter approach is somewhat supported by the general statements in paragraphs 92 & 93 of the current draft).
(b) The microscopic patterns of energy deposition in tissues and organs depends on the radiation field/particle type and energy and direction distributions at the location being considered.
(c) It is not in general meaningful, therefore, to determine a weighting factor for the relative effects of different types of radiation based on the radiation field/particle type and energy at a different location than that of the effect being considered.
(d) There is little substance to the argument that the effect being considered is the effect on the whole body as there are effectively no whole human RBE data, almost all data are for whole small animals or tissue cultures in vitro.
(e) It is always desirable to have a good general definition to cover all or most circumstances (Confucius in Analects xiii:3 considers that it is not possible to have any valid argument until rigorously defined terms are agreed). The definition of effective dose, and in particular the use of radiation weighting factors based on RBE values for radiation incident on the body is difficult to apply to complex high energy fields. (Consider stopping particles; Bragg peaks at different locations; particles producing a cascade of products in the body; etc. Accordingly, for space radiation fields, NCRP put the radiation weighting factor equal to the average quality factor in organ or tissue i.e. a return to effective dose equivalent)
(iii) It remains confusing to many radiation protection practitioners to have two quantities, dose equivalent and equivalent dose which are conceptually quite different.
Paragraph 89 – In the 3rd sentence: “The development of the quantity effective dose equivalent, and now effective dose, has made a significant contribution….” would be more accurate.
Paragraph 90 – in the 3rd line: it would be better to replace “measured” with “determined by instruments, or calculated,”. It is still contentious whether, strictly, the operational quantities can be measured.
Paragraph 95 - the last sentence is unnecessary.
Paragraph 105 – in the last sentence: the value 2 is incorrect. Both Figure 1 and equation 4.3 give a value of 2.5.
Paragraph 114 and Table 4 - this would benefit from a little more explanation of what wTs are and should perhaps comment on the main differences from Publication 60 values.
Paragraph 125 – in lines 4, 5, 7, 8 and 9: “d” should be in italics. Also, in line 4/5 soft tissue should be specified as ICRU 4-element tissue (see ICRU Report 39).
Paragraph 131 – the equation number in the last sentence should be 4.9 and not 1.9.
Paragraph 145 – as stated, collective doses are used as part of the optimisation process. This is often in a comparative way considering doses from different options or to different populations. It is not clear why this process should be restricted to collective effective dose and why collective equivalent dose to an organ or tissue should no longer be retained by ICRP. If a release of a mix of radionuclides to the environment contains elements, such as iodine, which give doses mainly to one organ or tissue then it can be useful also to estimate collective doses for this tissue. The breakdown of the collective effective dose may be different from that for the collective equivalent dose to the particular tissue, such as the thyroid.
Paragraph 147 – The statement that “collective dose is not intended as a tool for epidemiologic risk assessment and it is therefore inappropriate to use it in risk projections based on epidemiological studies” begs the question: what should be used in risk assessments? Clearly it would be helpful to use as detailed information as possible on the distribution of doses in an exposed population. Furthermore, there is undoubtedly greater uncertainty in risk projections based on very small doses delivered to a very large population than in assessments involving higher doses to a smaller population. However, for public health purposes, it would be helpful to clarify whether the argument for avoiding such calculations is based on uncertainties in risks at very low doses and/or difficulties in interpreting, comparing and communicating very small individual risks – whether from radiation or any other agent – that apply to large populations. As it stands, the recommendation that some types of risk calculation should be avoided might be taken to imply that there is no risk at very low doses, rather than that there may be very small risks at the individual level which – when summed over very large groups of people - may be difficult to communicate or to compare with non-radiation risks.
Paragraph 148 – this paragraph is recommending using limiting conditions for collective dose based on levels of individual dose. While this is obviously possible for occupational exposures where doses to individuals are known and summed to give the collective dose, in many cases it will not be possible for doses to the public. When assessing collective doses to the public from releases of radionuclides to the environment in many cases the ingestion of contaminated food is an important exposure pathway. As it is not possible to know exactly where everyone gets his or her food, collective doses from the ingestion pathway are generally based on the production of food and the distribution of individual doses are not known. If ICRP is going to recommend this use of limiting conditions it needs to specify exactly how this can be done for public doses and releases to the environment, as current methods are not available to do this integration between different individual doses.
Paragraphs 150, 151 and 152 – these have a seemingly incorrect use of the term precision. Precision and accuracy are not the same. Accuracy depends on trueness (inverse of bias or systematic measurement error) and precision (related to standard deviation). The accepted definitions (BIPM, ISO, IEC, IFCC, ILAC, IUPAC, IUPAP, OIML) are: accuracy (of measurement)- closeness of agreement between a measured quantity and a true quantity value of the measure and; precision- closeness of agreement between indications obtained by replicate measurements on the same or similar objects under stated specified conditions
Paragraph 153 – this paragraph is not clear, what existing values do not need re-computation? Perhaps an example, such as the derived exemption levels would help.
Section 5 – The system of radiological protection in humans
Particular questions raised by ICRP
This section is generally clear and the treatment of justification, optimisation and dose constraints is now adequate, subject to some specific comments below.
Justification of the upper dose constraint of 100 mSv (Paragraphs 202, 207, 208, 218 and 338)
When applied to total projected dose, either acute or over 1 year, 100 mSv is not a particularly high value. A one-off exposure of 100 mSv is very different from a continuing year on year exposure to 100 mSv. However, the ICRP Recommendations seem not to distinguish clearly between these two situations, but prefer to provide the same upper dose constraint for both emergency and existing exposure situations.
ICRP argues for 100 mSv on the basis of three claims (paragraph 202). Firstly, that the linear dose-response relationship is really only valid up to 100 mSv. Secondly, that above 100 mSv there is an increased likelihood of tissue reactions. Thirdly, that a significant excess of cancers have been identified in populations exposed to doses of around 100 mSv/y. Things may not be as clear cut as implied by ICRP and there is a danger that 100 mGy (mSv) will be seen as some sort of ‘practical threshold’ rather than recognising that it is difficult to confirm the existence of raised risks at very low doses. With regard to tissue reactions, this seems to be a change from the previous position where ICRP (e.g. in Publication 63) suggested
500 mSv/mGy as the threshold for deterministic injury. There is also the point that people may not be concerned about a tissue reaction that the individual is never aware of. With regard to observed cancers, does this apply to one-off exposures or just to continuing exposure at 100 mSv/y?
Paragraph 160 - the definition of a single source in this paragraph is not very clear. It could be ambiguous in the case of two discharge points on the same or adjacent sites. At what point is the dispersal sufficient for it to become a new source (eg nuclides in a lake) as opposed to the pipe or chimney as the point of discharge? What are the implications for additivity over pathways (this may be covered under paragraphs 181 and 182 but is still not very clear)?
Paragraph 161 - the reason why “the x-ray equipment in a hospital” should be given as an example of a ‘single source’, is not at all clear. Are they being considered as a source of public exposure (to the public inside or outside the hospital?) or occupational exposure? Why do you concentrate on the x-ray equipment in a hospital and disregard all the other radiation sources such as linear accelerators and the sealed and unsealed sources used in radiotherapy and nuclear medicine? This is a very poor example of a clear situation where the application of constraints to a ‘single source’ is appropriate or useful. It should be withdrawn.
Paragraph 164 – it would be helpful to clarify what is included in the category ‘medical exposures of patients’. In previous recommendations, ‘medical exposures’ included patients, comforters and carers, medico-legal exposures, volunteers in medical research etc. Are only patients included this time? If so, where is the protection of these other categories of exposed individuals going to be discussed?
Paragraph 167 - since public exposures are defined as those that are not occupational or medical, it would be more logical if this paragraph came after that defining medical exposures (paragraph 168).
Paragraph 168 - 1st sentence: radiation exposures of patients can also occur in interventional and guiding procedures and in radiotherapy treatment planning, which do not fall into the categories of ‘diagnostic, screening or therapeutic procedures’. It may be helpful to know that in the EC Medical Exposure Directive 97/43/Euratom and in UK regulations, to ensure inclusion of all these types of patient exposure, the phrase used is – “the exposure of patients as part of their own medical diagnosis or treatment”.
Last sentence: it would be helpful if this important point concerning fundamental differences in the way that radiation protection from medical exposures is dealt with in these recommendations was made more clearly and closer to the front of the report (see comment on paragraphs 28-33).
Paragraph 172 - what has this paragraph got to do with ‘the identification of the exposed individuals’?
Paragraph 173 - ‘Patients’ now appear to include members of the public acting as comforters and carers of patients. This should have been clarified at paragraph 164.
There needs to be a clear explanation of why the constraints for comforters and carers can be higher than for general members of the population? HPA-RPD welcomes the advice that there can be flexibility with regard to doses to carers so that they can choose to receive higher doses if they wish. However, the bald assertion that they ‘should’ be higher could be taken to imply that comforters and carers are of less value to society than other members of the public and consequently can be exposed to higher risks.
Paragraph 176 - it is important to confirm that the single individual (who can’t be representative) actually really is an isolated person in terms of habits and not just one of a small group with the others in the group not having been detected.
Paragraph 179 - it is unfortunate that the ICRP Committee 3 report on medical exposures is not yet available.
Paragraph 181 and 182 - shouldn’t these say ‘all regulated sources’?
Paragraph 183 and Figure 2 – in general the distinction between dose limits and constraints is much clearer than it was in the previous draft recommendations. However, it is still not clear whether doses from past controlled discharges should be included in the comparison with the dose limit. This is an issue that has caused confusion in the past and it is still not clear from the current draft. Figure 2 shows that the dose limit applies to all regulated sources in normal situations, which could be interpreted to include past controlled discharges. However, paragraph 186 states that the dose limit does not apply to existing situations and much then depends on the interpretation of the definition of existing situations in paragraph 162. It would be helpful if ICRP could give specific advice on this point.
Paragraph 185 – in the 2nd bullet point the avoidance of inequities and the use of dose constraints do not strictly apply to medical exposures (see comment on paragraph 30).
Paragraph 191 - it is not clear whether ICRP intends that the three levels of justification discussed in this paragraph should all be applied to all medical exposures of patients or whether there are situations in which only one or two of them should be applied (see comment on paragraph 250).
4th sentence: In justifying the medical exposure of a patient, why should any account be taken of incidental exposures of the radiological staff or members of the public? These occupational and public exposures need to be justified in their own right but should surely have no influence on the justification of the patient’s exposure.
5th sentence: “medical practitioner” means different things in different countries, so it might help to add “…. as defined in each countries national legislation.” to the end of the sentence. In the UK it is established practice to distinguish between healthcare practitioners who are medically qualified doctors and those who are not by referring to the former as ‘medical practitioners’ and the latter as ‘clinical practitioners’.
Paragraph 193 - as defined here, the principle of optimisation does not apply to medical exposures.
Paragraphs 198-210 - since none of this discussion on ‘Constraints’ applies to medical exposures, it would be better to point this out at the beginning rather than wait until paragraph 215.
Paragraph 215 - the first two sentences should be at beginning of section 5.8.1 and the second one would be better expressed as – “The optimisation of medical exposures has therefore to be dealt with differently from occupational and public exposures and is discussed in detail in chapter 6.”
Why are constraints for members of the public supporting patients discussed under the heading “Dose constraints in medical exposure of patients”? (See comment on paragraph 173).
Paragraph 220 – we welcome the flexibility regarding the use of dose constraints for emergency and existing exposure situations. However, it would be helpful if ICRP clarified whether the flexibility applies to the national value of the dose constraint or to the recommended upper dose constraint. If it is the latter then this needs underlining in a number of other places in the draft recommendations, where currently the upper dose constraint appears to be treated as an absolute upper bound.
Paragraphs 226 and 227 – these again make important points regarding optimisation and the use of the dose constraint.
Paragraph 230 – we agree with the point that collective doses can be used to examine the feasibility of an epidemiological study but this seems to contradict what is said in paragraphs 143 and 147.
Paragraph 231 – while the size of the exposed population indeed becomes more uncertain with time this might not be the case for individual doses from discharges to the environment. At long times following discharge doses will be mainly due to long-loved globally circulated radionuclides such as carbon-14. Doses from carbon-14 are mainly due to the consumption of food and as carbon is found in all foods the resulting doses are not particularly dependent on what people eat or where they get food.
Table 4 - the example given for the top dose category (‘Constraint for evacuation in a radiological emergency') seems to contradict other parts of ICRP advice. The Dose Constraint applies to projected doses and the sum of doses over all pathways resulting from a single source. Evacuation is a single countermeasure that addresses only some of the pathways of exposure. Surely, the appropriate type of example is ‘dose constraint for emergency planning to protect members of the public off-site for accidents occurring at nuclear sites’? Such an example meets the criterion of relating to all exposures resulting from a single source.
Paragraph 234 – in the 1st sentence: Planned situations surely include medical exposures of patients, where dose limits do not apply.